1. Field of the Invention
The complexity of the regulation of differentiation and proliferation of and by hematopoietic cells is becoming increasingly apparent as the list of factors which are isolated controlling these events continuously increases. For the most part, these factors are present in extraordinarily minute amounts in conjunction with numerous other proteins which serve a wide variety of functions. Factors which have been isolated and demonstrated to have activity include such polypeptides and proteins as .gamma.-interferon, platelet-derived growth factor, colony stimulating factor, interleukin-2, erythropoietin, as well as numerous other lymphokines. There is substantial interest in the isolation, purification and characterization of these blood components because of their possible use in treatment, as well as their use in elucidating such diseases as cancer.
There are many pitfalls in isolating a naturally occurring factor. A system of separation must be developed which separates the desired factor from other factors which are present and may have similar characteristics. Secondly, some means for assaying the various fractions must be provided which specifically or substantially specifically characterizes the material of interest, in contrast to the other materials which are present. Where the polypeptide of interest has extraordinarily high activity, the difficulty of isolating the desired product is greatly enhanced. Thirdly, one must provide procedures which do not detrimentally affect the product of interest, particularly avoiding any denaturation. In addition, there are frequently other materials in the composition which may act upon the material of interest, changing it, so that the product which is ultimately obtained, which may have some of the desired activity, is not the naturally occurring material. Finally, after isolating the desired component in sufficiently pure form, one must then attempt to physically characterize the polypeptide, for example, by amino acid sequencing, glycosylation number, disulfide bridges, and the like. One must further characterize the material as to its physiological characteristics.
Finally, even after isolation of the purified compound, characterization of the compound's physiological activity may often prove to be elusive. Since in many situations the activity may be dependent upon the presence of one or more other compounds, a narrow concentration range, particular host cells, or the like, substantial intuitive effort as well as extended experimentation is frequently required to discover a compound's physiological activity and demonstrate the parameters affecting its activity.
2. Description of the Prior Art
Holley et al., Proc. Natl. Acad. Sci. USA (1980) 77:5989-5992, describe the purification of epithelial cell growth inhibitors. Nelsen-Hamilton and Holley, ibid. (1983) 80:5636-5640, describe the effect of a growth inhibitor and epidermal growth factor in the incorporation of radiotagged methionine into proteins secreted by African green monkey cells (BSC-1). Morgan et al., Thromb. Haemost. (1980) 42:1652-60 provide the amino acid sequence for human platelet factor 4. Dawes et al., Thromb. Res. (1983) 29:569-81 and Schernthaner et al., Acta Med. Austriaca [suppl.] (1979) 6:375-9 report polyclonal antibodies to platelet factor 4. Lawler, Thromb. Res. (1981) 21:121-7 compares the sequences and structures of .beta.-thromboglobulin and platelet factor 4.